1. Field of the Invention
This invention relates to a magnetic recording medium, and more particularly, to an improvement in iron-base metal magnetic powder used in a magnetic recording medium of the coating type.
2. Prior Art
There is a desire for increasing magnetic recording density, and magnetic recording media meeting such requirements should have high coercivity and increased saturation magnetization. Then 8-mm video tape, DAT and similar magnetic recording media use in the magnetic layer iron-base metal magnetic powder having high coercivity and increased saturation magnetization and such media have been used in practice.
A variety of proposals have been made in order to establish higher coercivity and greater saturation magnetization. Exemplary are metal magnetic powders having silicon and other various compounds adsorbed to the surface (Japanese Patent Application Kokai (JP-A) Nos. 159904/1984, 162205/1984, 156209/1987, etc.).
These powders are prepared by adding an aqueous solution of a compound to a slurry of raw material particles such as .gamma.-Fe.sub.2 O.sub.3 or .alpha.-FeOOH and causing the raw material particles to adsorb the compound, followed by reducing reaction to form metal magnetic particles. When it is desired to adsorb more than one compound in a composite form, aqueous solutions of respective compounds are separately prepared and the compounds are separately applied to the raw material particles.
Also JP-A 140221/1978 discloses a method for preparing acicular crystal alloy magnetic particles by heat reducing acicular crystal hydrous or anhydrous ferric oxide particles containing Co or Co and Ni and coated with silicate on the surface and further forming a magnetite coating on the particle surface.
Further JP-A 161007/1980 discloses a method for preparing acicular crystal metallic iron magnetic particle powder by adding a water-soluble silicate to an aqueous solution containing Fe(OH).sub.2 at pH 11 or higher, effecting oxidation to form acicular crystal goethite particles, then converting them into acicular crystal hematite particles, and further heating and firing the particles in an atmosphere consisting of heated steam and non-reducing gas to form high density hematite particles, followed by heat reduction.
Further JP-A 306526/1988 discloses a metal magnetic powder based on a metal magnetic powder and/or iron nitride magnetic powder and containing at least one member selected from Al organic compounds, Ti organic compounds, Zr organic compounds, and Si organic compounds.
And JP-A 57701/1989 discloses metal magnetic powder which is prepared by separately applying Zr and Al compounds and a Ni and/or Cu compound to hydrous or anhydrous iron oxide, followed by reduction.
Further JP-A 257309/1989 proposes metal magnetic powder which is prepared by applying a Co compound or Co ion to hydrous or anhydrous iron oxide, followed by reduction.
Nevertheless, for example, the 8-mm video tape requires higher outputs and improvements in S/N and C/N, and further improvements in these properties are necessary when digital media are taken into account.
To this end, media should be increased in residual magnetization Br, increased in squareness ratio Br/Bm, reduced in switching field distribution (SFD), or improved in surface property of tape to reduce a spacing loss, and there is a need for improvements in these respects.